1. Field of the Invention
The present invention relates to an apparatus and a method for color image forming, and a program for the apparatus and the method, and more particularly to an apparatus and a method for color image forming capable of efficiently correcting displacement of an image in each of a plurality of colors, and a program for causing the apparatus to perform the method.
2. Discussion of the Background
In a conventional color image forming apparatus including four image forming devices, a full color image is formed by superimposing four toner images, each formed in one of four colors, by using one of the four image forming devices corresponding to the color.
When a full color image is formed, at least one of four toner images corresponding to one of the four colors may be out of color registration. When one of the four toner images in a corresponding color is displaced, for example, the displacement of the toner image in the color is corrected as described below.
When the toner image is displaced in a main scanning direction, the displacement of the toner image is corrected by logic control of the number of dots. Specifically, the amount of the displacement is measured, and the displacement is adjusted based on the amount of the displacement obtained by the measurement. In the adjustment, the amount of displacement is converted to a unit of dots. Then, a position in which a toner image in the color is to be formed is adjusted by the number of dots converted from the amount of displacement. By using the above method, adjustment with precision of 2 dots or less has been achieved.
Accordingly, when the toner image is displaced in a sub-scanning direction, the displacement of the toner image is corrected by logic control of the number of lines. Specifically, the amount of the displacement is measured, and the displacement is adjusted based on the amount of the displacement obtained by the measurement. In the adjustment, the amount of displacement is converted to a unit of lines. Then a position in which a toner image in the color is to be formed is adjusted by the number of lines converted from the amount of displacement. By using the above method, adjustment with precision of 2 lines or less has been achieved.
In general, when a color image forming apparatus is in practical use, displacement of a toner image in a corresponding one of a plurality of colors occurs over time. When a part of a laser optical system, an image forming unit, or the like is replaced, or maintenance is performed by disassembling the apparatus, the amount of displacement is significantly changed from an initial value. The amount of displacement after such maintenance is large compared with the amount of displacement observed in a normal operation. Therefore, measuring the amount of displacement before operation every time by using the same pattern results in a lack of precision, and the measurement may be ineffective.
A technology for correcting displacement to address the lack of precision has been disclosed. In a case of a color image forming apparatus automatically adjusting color registration, a pattern for measurement is formed as a unit of measurement, and the amount of displacement of an image in each of a plurality of colors is measured as a unit by using the pattern. Then, when the amount of displacement is large as in the case of displacement after the maintenance, the displacement is manually corrected. Alternatively, rough adjustment is firstly performed, and then fine adjustment is performed.